The invention is concerned with driving LEDs. It is normal for this purpose to use series resistors or current sources which limit and/or control the current through the LED. The LEDs are generally interconnected to form a cluster, that is to say a cluster comprises a series circuit of a plurality of LEDs. A plurality of LED clusters must be connected in parallel, that is to say be combined to form an array, depending on the size of the area to be lit or backlit. There is the basic problem here that a status terminal of the drive circuit is intended to supply a corresponding indication as soon as a fault has occurred in one or more LED clusters.
A first solution to this problem that is known from the prior art and comes from ST Microelectronics AG consists in interconnecting the entire LED array to form a single LED cluster. It is disadvantageous in this solution that such an LED cluster requires a substantially higher supply voltage in order to reach the LED cluster voltage, that is to say the sum of all the LED forward voltages. As soon as a fault occurs, the complete LED array is de-energized, that is to say it no longer shines.
A second solution that is known from the prior art and comes from Infineon Technologies AG consists in controlling and monitoring each individual LED cluster using a dedicated LED drive block. Since an LED array usually consists of a plurality of LED clusters, this invention is attended by the disadvantage that a plurality of LED driver blocks are required therefor. All the LED driver blocks are connected together to a single status terminal, and so it cannot be determined exactly how many LED clusters have failed. The use of a plurality of LED driver blocks is not desired, since this has a disadvantageous effect on the costs.
A further solution to the above problem, which is known from the prior art, is provided by the applicant of the present invention (DE19930174; Biebl) and functions as follows:
firstly, the principle of pulsed current control is explained with reference to FIG. 1 a series circuit of a plurality of LEDs, D1 to D4, is connected, on the one hand, to a supply voltage UBatt via a switch S1, and on the other hand to frame via a measuring shunt RShunt. The voltage UShunt dropping across the resistor is fed to an integrator 10 which provides at its output a mean value of the voltage present at the input. This voltage is fed to a controller 12 which also receives as input signal a reference voltage URef which corresponds to a mean desired value of the current ILed through the LEDs, D1 to D4. The control voltage URegel provided by the controller 12 at its output is supplied to the positive input of a comparator 14, there being present at its negative input a delta voltage UD which is provided by a triangle generator 16. The output signal of the comparator 14 is used to drive the switch S1. As emerges from the graph in the right-hand half of FIG. 1, the signal driving the switch S1 is a pulsed signal, recognizable from the squarewave function of the LED current ILED. This arrangement ensures that the current ILED flowing through the LEDs is controlled to a value correlated with the voltage URef.
Shown schematically and by way of example in the right-hand half of FIG. 2 are three such circuits, illustrated in FIG. 1, with pulsed current control, specifically the blocks 18, 20, 22. The supply voltage, the individual LEDs and, likewise, the resistors RShunt are omitted for reasons of clarity. If each of the blocks 18, 20, 22 comprises an LED cluster, an LED array can be realized by such an interconnection. A triangle generator 24 applies a clock signal 26 to a counter 28 which is applied to a multiplexer 30. The clock signal causes the multiplexer 30 to sample the control voltages of the three blocks 18, 20, 22 sequentially and feed them to a fault detection logic circuit with a comparator 32 and a flip-flop 34. As soon as one of the control voltages URegel 1, URegel 2, URegel 3 is lower than a prescribed threshold-value voltage USW, the comparator 32 generates a signal to the flip-flop 34 so as to generate at the Q output of the flip-flop 34 a signal which indicates a fault in one of the LED clusters of the blocks 18 to 22. Here, only the blocks 18 to 22 are mentioned by way of example, it being possible, of course, as indicated by the lines of dashes, for further blocks to be part of the same LED array.
The problem with this solution is, firstly, the additional outlay for a counter 28 and a multiplexer 30 and, on the other hand, the fact that a plurality of LED driver blocks are required in the case of larger LED arrays, since the number of current control loops per LED driver block is limited, for example to eight. The use of a plurality of LED driver blocks is reflected, in turn, disadvantageously in the price.
In addition to the disadvantages mentioned, in the case of the solutions addressed there is a further disadvantage that a fault signal is output immediately as soon as a fault has occurred. This is necessary, however, only if, as in the case of the first named solution, the complete LED cluster has failed. With regard to specific fields of application of LED arrays, for example in the vehicle sector as taillights, this would again justify the use of an incandescent bulb. In the case of an incandescent bulb, there also exists only two states of incandescent bulb intact and incandescent bulb not intact. The advantage of using LEDs in this sector resides, however, in the fact that in the event of failure of an LED cluster the light is capable of continuing to be operated if sufficient other functioning LED clusters are still present—although with somewhat diminished luminance—but, if suitably dimensioned, still above a limiting value prescribed by statute.